1. Field of the Invention
This invention relates to wireless communication systems and, more particularly, to antenna technology used in a radio frequency identification communication system.
2. Description of the Related Art
Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags or transponders. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. FIG. 1 illustrates a Modulated Backscatter (MBS) system. In a MBS system, after transmitting a message to the Tag (called the Downlink), the Interrogator then transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag then modulates the CW signal, using MBS, where the antenna is electrically switched, by the modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation. Modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink). Another type of RFID system uses an Active Uplink (AU). FIG. 2 illustrates an Active Uplink RFID system. In an AU system, the RFID Tag does not modulate and reflect an incoming CW signal, but rather synthesizes an RF carrier, modulates that RF carrier, and transmits that modulated carrier to the Interrogator. In some AU systems, the RF carrier used in the Uplink is at or near the same frequency as that used in the Downlink; while in other AU systems, the RF carrier used in the Uplink is at a different frequency than that used in the Downlink.
Conventional RFID systems are designed a) to identify an object passing into range of the Interrogator, and b) to store data onto the Tag and then retrieve that data from the Tag at a later time in order to manage inventory or perform some other useful application. In some RFID applications, directional antennas are used. For example, in an RFID-based electronic toll collection system, the Interrogator is overhung on top of the highway (see FIG. 3). In this application, the transmit and receive antennas have the same beamwidth. In fact, transmit and receive frequently share the same antenna, using a circulator to separate the transmit and receive paths.